Of the xylene isomers, para-xylene (PX) is of particular value since it is useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers. Para-Xylene is presently produced almost exclusively from naphtha by catalytic reforming process. The aromatics produced are used as feedstock to aromatics plants. Aromatics plants purify toluene and mixed xylenes. Purified toluene may be selectively or non-selectively disproportionated to produce para-xylene and benzene. Para-xylene may also be produced from mixed xylenes by isomerization/separations loop.
One known method for producing xylenes involves the alkylation of toluene with methanol over a solid acid catalyst. Thus the alkylation of toluene with methanol over cation-exchanged zeolite Y has been described by Yashima et al. in the Journal of Catalysis 16, 273-280 (1970). These workers reported selective production of para-xylene over the approximate temperature range of 200 to 275° C., with the maximum yield of para-xylene in the mixture of xylenes, i.e. about 50 wt % of the xylene product mixture, being observed at 225° C. Higher temperatures were reported to result in an increase in the yield of meta-xylene and a decrease in production of para and ortho-xylenes.
U.S. Pat. Nos. 7,119,239 and 7,176,339 disclose a process for the production of xylenes from reformate. The process is carried out by methylating under conditions effective for the methylation, the benzene/toluene present in the reformate outside the reforming loop, to produce a resulting product having a higher xylenes content than the reformate. Greater than equilibrium amounts of para-xylene can be produced by the process. U.S. Pat. No. 7,186,873 discloses process for the production of xylenes from reformate by reactive distillation. The process is carried out by methylating the benzene/toluene present in the reformate in a reactive distillation zone and under reactive distillation conditions to produce a resulting product having a higher xylenes content than the reformate. Greater than equilibrium amounts of para-xylene can be produced by the process.
As far as the present inventor is aware, it has previously not been suggested to methylate a pyrolysis gasoline (pygas), to manufacturing para-xylene nor has been suggested to integrate a catalytic system comprising a steam cracker and a methylation unit to produce para-xylene, and optionally co-produce light olefins. There is no prior art disclosing the production of para-xylene from resid and methylating agent feedstocks by any means. There is no prior art teaching the use of methylation technology to convert ethylbenzene to para-xylene and ethylene.
U.S. Pat. No. 6,046,372 discloses a process for converting methanol and/or dimethyl ether to a product containing C2 to C4 olefins which comprises the step of contacting a feed which contains methanol and/or dimethyl ether with a catalyst comprising a porous crystalline material, said contacting step being conducted in the presence of an aromatic compound under conversion conditions including a temperature of 350° C. to 480° C. and a methanol partial pressure in excess of 10 psia (70 kPa), said porous crystalline material having a pore size greater than the critical diameter of the aromatic compound and the aromatic compound being capable of alkylation by the methanol and/or dimethyl ether under said conversion conditions.
U.S. Pat. No. 6,680,418 discloses a process for converting methanol and/or dimethyl ether to a product containing C2 to C4 olefins, which comprises the step of contacting a reaction mixture containing methanol and/or dimethyl ether and at least 10 wt % of a polymethylbenzene component selected from trimethylbenzenes, tetramethylbenzenes and mixtures thereof with a catalyst comprising a porous crystalline material. The contacting step is conducted under conversion conditions including a temperature of about 250° C. to about 500° C. and a methanol and/or dimethyl ether partial pressure of about 5 to about 250 psia (35 to 1725 kPa). The porous crystalline material used in the catalyst has a pore size greater than the critical diameter of the aromatic compound and a Diffusion Parameter for 2,2-dimethylbutane of at least 500 sec−1 when measured at a temperature of 120° C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa).
All methylation processes use relatively pure toluene, mixtures of toluene and benzene, or reformate as feedstock for manufacturing xylene, preferably para-xylene. The sources of toluene/benzene feedstock for the methylation process may be reformate or pygas. However, to the best knowledge of the inventor, there is no disclosure of using pygas as the aromatic feedstock for the methylation process. It is well known to a person of ordinary skill that pygas contains large amount of non-aromatics, some olefinic hydrocarbons, and a high ethylbenzene content in the C8 aromatics. Both non-aromatics and olefinic hydrocarbons are known for catalyst deactivation and aging. Ethylbenzene is known as a bad feedstock for para-xylene production. Furthermore, there is no disclosure using pygas without extraction as the aromatic feedstock for the methylation process. Even more, there is no disclosure using pygas obtained from steam cracking a hydrocarbon feedstock made with hydroprocessed resid with/without extraction as the aromatic feedstock for the methylation process.
There is, therefore, a need to develop a technology to produce para-xylene in high yield from pygas feedstock. Another technical challenge related to use of pygas feedstock for para-xylene production is rapid catalyst deactivation because the pygas feedstock contains certain amount of non-aromatics which may deactivate the catalyst. Therefore the pygas feedstock normally is subjected to an extraction process to remove most of the non-aromatics before further processing. However, the extraction process is expensive and capital intensive. There is also a need to produce aromatic products from pygas without extraction.
We surprisingly discovered a process of manufacturing PX, optionally co-manufacturing light olefins, from a pygas feedstock by methylating the pygas with a methylating agent using a molecular sieve catalyst. Furthermore, the para-xylene is produced with higher than thermodynamic equilibrium amount when a selectivated molecular sieve is used as catalyst for the methylation step.